The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving ways, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. As shown in FIG. 1, the most commonly used AMOLED pixel driving circuit comprises two thin film transistors (TFT) and one capacitor, i.e. a 2T1C pixel driving circuit. Specifically, the 2T1C AMOLED pixel driving circuit comprises a first thin film transistor T1, a second thin film transistor T2 and a capacitor C. The first thin film transistor T1 is a switching thin film transistor, and the second thin film transistor T2 is a driving thin film transistor, and the capacitor C is a storage capacitor. A gate of the first thin film transistor T1 is electrically coupled to a scan signal GN, and a source is electrically coupled to a data signal SN, and a drain is electrically coupled to a gate of the second thin film transistor T2 and one end of the capacitor C; a drain of the second thin film transistor T2 is electrically coupled to a high driving voltage OVDD, and a source is electrically coupled to an anode of an organic light emitting diode D; a cathode of the organic light emitting diode D is electrically coupled to a low driving voltage OVSS′; the one end of the capacitor C is electrically coupled to the drain of the first thin film transistor T1, and the other end is electrically coupled to the drain of the second thin film transistor T2. When the AMOLED displays, the scan signal GN controls the first thin film transistor T1 to be on, and the data signal SN enters the gate of the second thin film transistor T2 and the capacitor C through the first thin film transistor T1, and then the first thin film transistor T1 is off. With the storage function of the capacitor C, the voltage of the gate of the second thin film transistor T2 still can keep the voltage of the data signal to make the second thin film transistor T2 in an on state. The driving current Ioled enters the organic light emitting diode D through the second thin film transistor T2 to drive the organic light emitting diode D to emit light. The illumination intensity of the organic light emitting diode D is related with the driving current Ioled passing through the organic light emitting diode D. The driving current Ioled is influenced by the voltage difference ΔVoled between the anode and the cathode of the organic light emitting diode D. As shown in FIG. 2, with the increase of the ΔVoled, the Ioled also constantly increases, and ΔVoled=Vs−OVSS, wherein Vs is the voltage of the source of the second thin film transistor T2, and the OVSS is the low driving voltage, and the power consumption of the organic light emitting diode D is that P=Ioled×ΔVoled. Therefore, as shown in FIG. 3, with the increase of the Ioled, the intensity Lum of the organic light emitting diode D also gradually increases.
With the constant development of the OLED display technology, the consumers have higher and higher demands to the display quality of the OLED display panel. The contrast of the OLED display panel required to be promoted in advance to raise the display quality of the OLED display panel. Generally, the prior art is to directly implement contrast enhancement treatment to the image in the RGB space model. Such treatment can easily generate the defect of losing colors and HSI is a color space model established according to the intuitive features of the colors. The HSI color space is developed from based on the visual system of the human. The hue H, the saturation S and the intensity I are employed to describe the color, and can clearly express the variations of the hue H, the saturation S and the intensity I.